Turbine components must maintain physical and thermal properties for useful applications. Turbine components are subject to high temperatures, and thus are readily oxidized. Turbine components are also subject to high stresses during operation that often lead to creep (deformation under a steady load, especially at elevated temperatures) of the turbine's material. Turbine components therefore should be formed from a material that maintains its mechanical properties, such as, but not limited to, enhanced creep resistance and lack of embrittlement, and does not readily oxidize at elevated temperatures.
Turbine components are often formed from steel materials. Steels exhibit excellent strength, low brittle to ductile transition temperatures and good hardening characteristics. Steels, however, are subject to oxidation, embrittlement, and creep on exposure to elevated temperatures. The embrittlement is due, at least in part, to formation of detrimental phases within alloy grains (irreversible embrittlement) or to segregation of some harmful elements to grain boundaries (reversible embrittlement) at elevated temperatures. Steels for turbine component applications must be formed with constituents that reduce steel embrittlement, oxidation and creep.
Conventional steel alloys for turbine components include high alloy steels. High alloy steels include steels with a chromium (Cr) content above 10%, for example about 12% by weight percent. High alloy steels include, but are but not limited, to Fe--12Cr stainless steels (hereinafter Fe--12Cr steels), which are known in the art. One such steel is disclosed in U.S. Pat. No. 5,320,687 to Kipphut et al., the entire contents of which are fully incorporated herein by reference.
Common steel alloying constituents comprise, but are not limited to, tungsten (W) and cobalt (Co). For example, an addition of tungsten to a steel requires either (1) a decrease in a chromium (Cr) content to maintain a balance of ferrite stabilizers in the steel; or (2) additional austenite stabilizers, such as, but not limited to, nickel (Ni), manganese (Mn), and cobalt, to maintain an adequate steel oxidation resistance. Since most austenite stabilizers are expensive (cobalt) or detrimental to creep properties (nickel), an austenite stabilizer addition does not maintain a steel's oxidation and creep resistance. Steel manufacturers thus have attempted to decrease the chromium content in steels for turbine components. A low chromium content does not add much cost to the manufacture of the steel, and does not adversely effect creep properties. A low chromium content in steel, however, is detrimental to oxidation resistance, and is undesirable.
Further attempts to solve a steel's oxidation resistance problems include addition of one or both of chromium and silicon (Si). Chromium and silicon are added to enhance oxidation resistance of steels, which, of course, is desirable. These solutions, however, have not proved effective or desirable as a higher chromium content, while enhancing oxidation resistance, undesirably increases embrittlement in steels by an alpha prime (.gamma.) phase formation. Also, the silicon addition romotes formation of undesirable, embrittling Laves phases in steels.
Accordingly, it is desirable to provide a steel composition that provides suitable performance in high temperature applications, with balanced mechanical and oxidation properties. For example, a steel for high temperature turbine components applications should exhibit reduced oxidation, while balancing desirable mechanical properties, such as enhanced creep resistance and reduced embrittlement at high temperatures.